FZU 30.5.061 Comparison of Mn doped GaAs, ZnSe, and LiZnAs dilute magnetic semiconductors J.Mašek, J. Kudrnovský, F. Máca, and T. Jungwirth.

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Presentation transcript:

FZU Comparison of Mn doped GaAs, ZnSe, and LiZnAs dilute magnetic semiconductors J.Mašek, J. Kudrnovský, F. Máca, and T. Jungwirth

FZU Introduction: A II B VI vs. A III B V DMS New proposition: Li(Zn,Mn)As Basic picture: scaling of Tc in (Ga,Mn)As Comparison of (Zn,Mn)Se, (Ga,Mn)As, and Li(Zn,Mn)As based on density functional calculations Li(Zn,Mn)As – a detailed study Conclusions

FZU (A II,Mn)B VI DMS (Cd,Mn)Te, (Zn,Mn)Se Isovalent substitution of Mn(A II ) Mn atoms neutral, with spin 5/2 Wide concentration range of alloys Intrinsic semiconductors (may not be the case for other transition metals)

FZU (A II,Mn)B VI DMS … cont. Giant AF splitting of the valence band: E v (↑)>E v (↓) Antiferromagnetic exchange coupling of local moments (superexchange) Complicated phase diagram due to frustration … semimagnetic semicond. Ferromagnetic state achieved only in strongly n- type doped materials

FZU (A III,Mn)B V DMS (Ga,Mn)As, (Ga,Mn)N Non-isovalent substitution of Mn(A III ) Limited range of alloying (x<0.1) Mn acts as single acceptor Without extra doping, (Ga,Mn)As is p-type semiconductor (E F ~ E v – 0.2eV) Local moment 5/2 + hole spin -1/2

FZU (A III,Mn)B V DMS … cont. Strong ferromagnetic RKKY interaction of local moments due to free carriers Ferromagnetic state with T c <170K (T c increases with both x and h) Superexchange seems unimportant and is usually neglected Troubles with selfcompensation (Mn ( int ) )

FZU Open problems Limitations of T c Dependence of T c on x and density of carriers from the microscopic theory The role of superexchange in (Ga,Mn)As Possibility of n-type FM semiconductors Optimization of the host materials

FZU Proposition: Li(Zn,Mn)As Band structure of LiZnAs similar to ZnSe and GaAs a(LiZnAs)= 5.815Å, a(GaAs)= Å → hybridization Easy substitution of Mn for Zn expected Number of carriers related to non-stoichiometry in Li sublattice (vac (Li) =acceptor, Li (int) =donor)

FZU Basic picture sp 3 bands + Anderson Hamiltonian for 3d 5 -states of Mn Spin-orbit interaction neglected CPA description of the mixed crystal in the FM state E rev ≡ energy of flipping a single atomic moment In mean-field approximation, T c ~ E rev.

FZU

FZU Preliminary results for (Ga,Mn)As General scaling rule: T c /x ≈ f(n/x) T c <0 for |n|<x/4 … superexchange important ! Saturation of T c for n ≈ -x FM state (T c >0) possible for both p-type and n- type !

FZU Detailed density-functional study LMTO-CPA, LDA (partly LDA+U) Supporting calculations: FP LAPW Densities of states & distribution of Mn d-states in (Zn,Mn)Se, (Ga,Mn)As, and Li(Zn,Mn)As Mean-field T c in rigid-band approximation T c in realistic, co-doped DMS

FZU DOS: (Ga,Mn)As vs. (Zn,Mn)As

FZU DOS: (Zn,Mn)Se vs. Li(Zn,Mn)As

FZU Curie temperature: rigid-band approx.

FZU Curie temperature: co-doped DMS

FZU

FZU Summary Dependence of T c (n) similar in all (Ga,Mn)As, (Zn,Mn)Se, and Li(Zn,Mn)As: no difference of II-VI and III-V DMS Superexchange → unstable FM state for low concentration of carriers n-type ferromagnetic DMS available at high concentration of donors Seems realistic only in Li(Zn,Mn)As

FZU Li(Zn,Mn)As – more details Problems: Microscopic nature of Li non-stoichiometry (vac (Li) and Li (int), Li (Zn) and Zn (Li) ) Limits of solubility of Mn in LiZnAs Mn solubility in non-stoichiometric material

FZU Substitution of Mn for Zn not affected by non-stoichiometry

FZU Large formation energies of vac (Li) and Zn (Li) → substoichoimetry unprobable

FZU Both Li (int) and Li (Zn) possible, leading to either n- or p-type material

FZU Dynamical equilibrium Principal defects in Li(Zn,Mn)As: Mn (Zn), Li (Zn),and Li (int) Formation energies E s,i (x s,x i ) of Li (Zn) and Li (int) as functions of partial concentrations x s and x i. Balanced state: E s (x s,x i ) = E i (x s,x i ).

FZU

FZU Partial concentrations of Li (Zn) and Li (int)

FZU Conclusions Both p-type and n-type ferromagnetic DMS possible with suitable doping of III-V, II-VI, and I-II-V DMS AFM exchange in compensated materials Electron-mediated FM: disorder-induced reconstruction of CB + partial occuparion of side minima + strong admixture of d-states (unclear) Li-rich Li(Zn,Mn)As seems a good candidate for n-type FM semiconductor.